With the rapid development of information technology, electrical document are widely used. However, for the purpose of storage, processing and delivery via internet network of the documents, it is necessary to scan the paper document into the format of electrical documents, particularly, for the intention of clarity of the documents while delivering these documents. Therefore, it is quite important to ensure that the scanning quality of the paper documents meets the requirement. Conventionally, it needs a period of warm-up time before the scanner starts to operate. When the scanner scans a plurality of pages of the document, the lamp brightness of the scanner is gradually increased with the increment of the scanned pages. Thus, there is a brightness difference between the current page and the previous page so that the scanning quality of the pages of the document is affected since the scanning brightness for each page is not uniform. For example, the scanning brightness provided by the lamp for the last page is considerably greater than that of the first page of the document.
FIG. 1 is a schematic block diagram of a conventional scanner 100 with brightness compensation. The scanner 100 includes a lamp 102 an image sensor 104, an analog-front-end (AFE) 106, an image processor 110, a direct-current (DC) voltage control unit 112 and an inverter 114. The image processor 110 further includes an image processing module 116. The lamp 102 provides light source to illuminate the page of the document. The image sensor 104 senses the image data on the pages for outputting the analog image data. The analog-front-end (AFE) 106 converts the analog image data into the digital image data. The image processor 110 processes the digital image data by using the image processing module 116. The direct-current (DC) voltage control unit 112 outputs the DC voltage. The inverter 114 is coupled to the DC voltage control unit 112 for receiving the DC voltage and outputs a constant DC voltage to control the brightness of the lamp 102. However, the image processor 110 cannot adjust the image gain of the digital image data using the image processing module 116 so that the scanned brightness on the page is not uniform. In addition, the DC voltage control unit 112 and the inverter 114 use constant voltage to drive the lamp 102 but do not receive the control information from the image processor 110 so that the illuminance of each page is variable disadvantageously, thereby downgrading the scanning quality of the document. Consequentially, there is a need to develop a novel scanning technique to solve the aforementioned problems.